Power tools provided with a rectifier circuit for rectifying AC voltage supplied from an AC power supply, and a large-capacitance smoothing capacitor for smoothing the voltage outputted from the rectifier circuit into a substantially constant DC voltage have been widely used. Since the smoothing capacitor in this type of power tool has a large capacitance, the periods in the cycle of the AC voltage in which the absolute value of voltage from the AC power supply is higher than the voltage across the terminals of the smoothing capacitor are short and, thus, the periods in which current flows from the AC power supply to the power tool are short. Consequently, the waveform of the current flowing into the power tool from the AC power supply is distorted and has many harmonics, lowering the power factor.
To resolve the problem described above, there is known in the art a control device for controlling the inverter that drives the motor (Patent Literature 1). The control device attempts to improve the power factor by using a small-capacitance smoothing capacitor to lengthen the period during which current flows from the AC power supply to the power tool. Since the smoothing capacitor in this control device is configured with a small capacitance, the capacitor only slightly smooths voltage that has undergone full-wave rectification in the rectifier circuit, and thus voltage having a near-full-wave rectified waveform (hereinafter called a “near-full-wave rectified voltage”) is applied to the brushless motor.
The structure described above for applying voltage having a near-full-wave rectified waveform to the brushless motor may produce torque ripple because, during the variation cycle of the near-full-wave rectified voltage, electric current does not flow through the coils during periods in which the near-full-wave rectified voltage is lower than the induced voltage generated in the coils of the brushless motor. To suppress this torque ripple, the conventional control device performs field-weakening control for reducing the induced voltage. By this field-weakening control, the period in which the near-full-wave rectified voltage is lower than the induced voltage is shortened and thus the period in which electric current does not flow through the coils is also shortened, thereby suppressing torque ripple.